The FCM approach for analyzing cellular BrdUrd content and cell cycle position has become an extremely popular method for investigating cell proliferation under a variety of experimental conditions. The monoclonal anti-BrdUrd technique remains the most widely used method. We have developed a method in our laboratory that utilizes differential fluorescence of the DNA fluorochromes Hoechst 33342 and mithramycin. In principle the method sensitively measures the quenching of Hoechst by BrdUrd incorporated into cells by brief treatment with the thymidine analog in culture. Recently Darzynkiewicz and co-worker developed a new BrdUrd analysis method called "Strand Breaks Induced by Photolysis" (SBIP) since breaks in the DNA of BrdUrd-labeled cells are induced by uv exposure. The 3' Oh termini at the sites of DNA cleavage are labeled with biotin- or digoxygenin-conjugated deoxynucleotides in a reaction catalyzed by exogenous terminal deoxynucleotidyl transferase and fluoresceinated avidin- or digoxygenin-antibodies are then used to label the strand breaks. In a series of rigorous FCM studies we will carefully examine the various methods for their ease of usage, their accuracy and reproducibility.